Brazing is a process in which parts are joined with the aid of a filler metal which has a melting point below that of the materials being joined. Brazing has been used to join metallized ceramics to metal or to other ceramics by creation of brazed joints.
Heretofore, metallized coatings have been formed on ceramics by a variety of techniques. One of the well known techniques is the application of molybdenum/manganese paints which are applied to the ceramic article to be metallized and subsequently fired at a high temperature, such as 1200.degree. C., in a wet reducing atmosphere to convert the metal oxides to a metallized layer. In general, this is an example of a sintered metal powder process for forming a metallized ceramic.
Metallized coatings can also be deposited by chemical vapor deposition or by a physical vapor deposition. Among the physical vapor deposition processes employed have been sublimation and evaporation, sputtering and ion plating. See, for example, "Joining Ceramics and Graphite to Other Materials", NASA SP-5052, Office of Technology Utilization, National Aeronautics and Space Administration, pp 20-27 (1968).
Recently, however, applications for brazed joints employing metallized ceramics have developed requiring better high-temperature properties than has heretofore existed with metallized ceramics. High-temperature solar receivers, which convert solar energy to heat and drive a gas turbine to produce electrical power, are an example of such an application. Heat recovery systems in nuclear plants, coal-fired plants or coal gasification plants are other applications which have heretofore been limited in operating temperature, partially because of the lack of brazed joints between metallized ceramics and metals or other ceramics at higher temperatures. In recognition of this, a large amount of research has been conducted directed towards finding or developing metallized ceramics having outstanding properties at high temperatures.